Synoptic Discussion - March 2014
Note: This Synoptic Discussion describes recent weather events and climate anomalies in relation to the phenomena that cause the weather. These phenomena include the jet stream, fronts and low pressure systems that bring precipitation, high pressure systems that bring dry weather, and the mechanisms which control these features — such as El Niño, La Niña, and other oceanic and atmospheric drivers (PNA, NAO, AO, and others). The report may contain more technical language than other components of the State of the Climate series.
The long-wave circulation pattern for March 2014 over North America was a continuation of the overall pattern seen throughout the winter, but with some changes. It consisted of a dominant upper-level trough over eastern Canada which extended into western Canada and the northeastern U.S., thus bisecting the upper-level ridge over western North America. Short-wave troughs undercut the western ridge, allowing Pacific weather systems to penetrate into the Pacific Northwest. This long-wave pattern reflects the influence of atmospheric modes of variability (East Pacific-North Pacific and West Pacific) associated with the North Pacific driver, whose teleconnections include colder-than-normal temperatures east of the Rockies and warmer-than-normal temperatures in the Southwest, although signs of the influence of other modes of circulation were evident in the precipitation anomaly pattern. The combination of cold and moist air masses produced frequent winter storm systems east of the Rockies, while the Pacific weather systems that broached the western ridge brought mostly rain in the warmer air over the West. The March precipitation was not enough to eliminate seasonal deficits, and the crucial mountain snowpack remained deficient in many areas, so drought conditions continued across much of the West, although the precipitation brought improvement to some areas, especially the Northwest. The western ridge continued in force over the Southwest into the Southern Plains, where the lack of precipitation saw expansion of drought. See below for details.
In the Northern Hemisphere, March is the beginning of climatological spring which is the time of year when solar heating forces the jet stream and circumpolar vortex to contract poleward. Cold polar air masses can still influence the weather, but warm southerly air masses begin to move into the contiguous United States (CONUS) more frequently. In March 2014, however, a strong long-wave trough in the upper-level circulation remained entrenched over eastern Canada. The northerly flow over central North America, associated with the trough, directed cold northerly air masses into the CONUS.
This caused Canadian cold fronts to frequently charge into the U.S. east of the Rockies all the way to the Gulf of Mexico coast and beyond. This northerly flow of cold dry air masses kept much of the central and eastern U.S. colder than normal for the month, resulting in top ten coldest March state rankings concentrated in the Great Lakes and Northeast, and keeping nearly two-thirds of the Great Lakes frozen into early April. With colder-than-normal monthly temperatures affecting the high population centers of the Midwest and Northeast, the national REDTI (Residential Energy Demand Temperature Index) ranked as the 26th highest for March.
The circulation pattern also deflected much of the Gulf of Mexico and Atlantic moisture away from the interior CONUS, resulting in a drier-than-normal month from the Great Plains to Central Appalachians and some top ten driest March state rankings. A long-wave ridge in the upper atmosphere dominated the southwestern U.S., bringing warmer-than-normal temperatures and inhibiting widespread precipitation. This combination of northerly flow in the central U.S. and ridge over the Southwest caused drought to expand in the Plains, Midwest, South, Southeast, Southwest, and U.S. as a whole, and the intensity of drought to increase significantly in the Southern Plains.
Meanwhile, energetic short-wave troughs moving in the upper-level flow picked up Pacific moisture and slammed into the western CONUS, bringing precipitation to the Pacific Northwest, Northern Rockies, and northern California. The rain and snow contracted drought in the Northwest, but expansion and intensification of drought in the Southwest resulted in a net small increase of drought for the West. The short-wave troughs had enough energy and moisture left in them, after traversing the Rockies, to generate surface low pressure systems which, when combined with the below-freezing temperatures, blanketed the northern tier states with a persistent and heavy snow cover, continuing a pattern that has occurred for much of the winter.
The first week of March saw the greatest surge of colder-than-normal air and winter snow storms which created the most extensive snow cover of the month. The circulation at higher latitudes (Canada) relaxed and became more zonal at mid-month, with warmer temperatures (weeks 2 and 3) and less extensive snow cover becoming common over the CONUS. March went out like a lion during the last week as the upper-level circulation again became more northerly at high latitudes, sending cold air masses surging back into the CONUS, dropping temperatures, and triggering a round of severe weather, including several tornadoes. The northerly flow and relatively dry air masses kept the number of tornadoes low — the preliminary count of 25 tornadoes was well below the average count of 80 tornadoes for March. However, the dry and windy weather that accompanied the cold fronts set the stage for a surge of large wildfires near the end of the month in the central to southern Plains and southern Appalachians.
By the end of March, moderate to exceptional drought affected 38.4 percent of the CONUS (according to the U.S. Drought Monitor). There were five times as many record cold daily maximum and minimum temperatures (5822) as record warm daily maximum and minimum temperatures (1149). The unusually cold maximum and minimum temperatures caused by the persistent upper-level trough, combined with fewer-than-normal days with precipitation, but heavier precipitation events when it did rain, resulted in the the biggest CEI (Climate Extremes Index) for March in the Northeast. A different combination of factors generated the seventh largest March CEI for the West — unusually warm maximum and minimum temperatures and intense, widespread long-term drought associated with persistent upper-level ridging, and heavy 1-day precipitation events caused by extreme weather systems which penetrated the ridge. The combination of these diverse regional extremes, caused by opposing weather factors, gave the CONUS the ninth largest CEI for March — largely due to cold minimum temperatures but also to widespread drought and heavy 1-day precipitation events.
When integrated across the month, the circulation produced a pattern of below-normal 500-mb heights (stronger-than-normal long-wave trough) over eastern North America which extended into the north central and northeastern CONUS and the Canadian Prairies. Above-normal 500-mb heights extended over northern Alaska and northwest Canada, as well as the southwestern CONUS extending into the adjacent Pacific Ocean, reflecting a bisected western long-wave ridge, much like last month.
The Pacific storm track brought above-normal precipitation to the Pacific Northwest and Northern Rockies, while short waves moving along southerly fronts moistened areas along the Gulf of Mexico and Atlantic coasts. Although precipitation did fall in the interior CONUS, monthly totals were below normal across the Plains, Midwest, and parts of the Great Lakes, Northeast, and Southeast, as well as in the Southwest beneath the upper-level ridge. With the Pacific storm track hammering the Pacific Northwest, most weather systems were directed south of Alaska, resulting in a drier-than-normal month for much of the state.
The pattern of monthly temperature anomalies over the CONUS followed the upper-level circulation pattern — warmer than normal beneath the southwest ridge and cooler than normal east of the Rockies beneath the upper-level trough. Temperatures in Alaska averaged warmer than normal in the north, under the influence of above-normal upper-level heights, and cooler than normal in the southeast.
The upper-level circulation over North America in March is an example of an interconnected and amplified meridional pattern. The atmospheric circulation is interconnected around the world, and this interconnectedness was manifested this month by a pattern of paired above-normal and below-normal 500-mb height anomalies over North America, the North Atlantic, and Eurasia. A dipole of below-normal heights can be traced from the high latitudes over north central Asia, extending across the Arctic into lower latitudes over eastern North America. Above-normal heights dominated much of Europe and Asia. This was reflected at the surface over Eurasia as widespread above-normal temperatures and below-normal snow cover, with areas of below-normal precipitation. The atmosphere compensated for the Eurasian warmth by extending the circumpolar vortex over North America with its accompanying cold and snowy weather.
Subtropical highs, and cold fronts and low pressure systems moving in the storm track flow, are influenced by the broadscale atmospheric circulation. The circulation of the atmosphere can be analyzed and categorized into specific patterns. The tropics, especially the equatorial Pacific Ocean, provides abundant heat energy which drives the world's atmospheric and oceanic circulation. The following describes several of these modes or patterns of the atmospheric circulation, their drivers, the temperature and precipitation patterns (or teleconnections) associated with them, and their index values this month:
El Niño Southern Oscillation (ENSO)
- Description: Oceanic and atmospheric conditions in the tropical Pacific Ocean can influence weather across the globe. ENSO is characterized by two extreme modes: El Niño (warmer-than-normal sea surface temperature [SST] anomalies in the tropical Pacific) and La Niña (cooler-than-normal SST anomalies), with the absence of either of these modes termed "ENSO-neutral" conditions.
- Status: Ocean temperatures and atmospheric circulation anomalies indicated that the equatorial Pacific continued in an ENSO-neutral state during March, but SST anomalies were warming.
- Teleconnections (influence on weather): To the extent teleconnections are known, while in a neutral state, ENSO normally is not a player in the month's weather. Historical data has been analyzed by NOAA to show typical temperature and precipitation patterns associated with El Niño and La Niña ENSO episodes. Teleconnections are not available for ENSO-neutral conditions at that NOAA web site.
Madden-Julian Oscillation (MJO)
- Description: The MJO is a tropical disturbance or "wave" that propagates eastward around the global tropics with a cycle on the order of 30-60 days. It is characterized by regions of enhanced and suppressed tropical rainfall. One of its indices is a phase diagram which illustrates the phase (1-8) and amplitude of the MJO on a daily basis. The MJO is categorized into eight "phases" depending on the pattern of the location and intensity of the regions of enhanced and suppressed tropical rainfall. The MJO can enter periods of little or no activity, when it becomes neutral or incoherent and has little influence on the weather. Overall, the MJO tends to be most active during ENSO-neutral years, and is often absent during moderate-to-strong El Niño and La Niña episodes.
- Status: The MJO started the month in phase 8, then transitioned to phases 1 and 2 as the month progressed. But the MJO indices appeared to be strongly influenced by slowly evolving atmospheric and oceanic conditions in the western Pacific which confused and, as the month progressed, worked against and weakened the MJO signal (as indicated by the MJO discussions for March 3, 10, 17, 24, and 31).
- Teleconnections (influence on weather): The MJO's temperature and precipitation teleconnections to U.S. weather depend on time of year and MJO phase. To the extent teleconnections are known, the February-April teleconnections for precipitation are shown here and for temperature are shown here.
- Comparison to Observed: The MJO is transitory and can change phases (modes) within a month, so it is more closely related to weekly weather patterns than monthly. The March 2014 monthly precipitation anomaly pattern appears to match that expected with MJO phases 8 and 1 in the Northern Plains and interior Southeast (drier than normal), with the match reasonably good during the first half of the month (weeks 1 and 2). The monthly temperature anomaly pattern does not match the teleconnections for MJO phases 8 and 1 where the correlations should be the greatest (Northern Plains and Northwest), but there is some correlation in the Northern Plains at mid-month (week 2).
- The Pacific/North American (PNA) pattern
- Description: The PNA teleconnection pattern is associated with strong fluctuations in the strength and location of the East Asian jet stream. PNA-related blocking of the jet stream flow in the Pacific can affect weather downstream over North America, especially the West and especially in the winter half of the year.
- Status: The daily PNA index was slightly positive for the first half of the month then slightly negative for the last half, averaging weakly negative for the month as a whole.
- Teleconnections (influence on weather): To the extent teleconnections are known, the temperature teleconnection map for a negative PNA at this time of year (April on the maps) shows below-normal temperatures in the Pacific Northwest, Alaska, and most of Canada, and above-normal temperatures across the Southern Plains to Southeast. The precipitation teleconnection map for this time of year shows wetter-than-normal conditions in the Upper Mississippi Valley and drier-than-normal weather in Colorado to Kansas. The upper-level circulation anomaly teleconnection for a negative PNA shows below-normal heights over most of Canada and above-normal heights over the southeastern third of the CONUS. A positive PNA is associated with the opposite patterns.
- Comparison to Observed: The March 2014 temperature anomaly pattern does not agree with that expected with a negative PNA, while there is some agreement in the precipitation anomaly pattern in Colorado to Kansas. The upper-level circulation anomaly pattern shows some some agreement over Canada, although not where the teleconnections are greatest, and no agreement over the CONUS.
- The Arctic Oscillation (AO) pattern
- Description: The AO teleconnection pattern relates upper-level circulation over the Arctic to circulation features over the Northern Hemisphere mid-latitudes and is most active during the cold season.
- Status: The daily AO index began the month neutral (near zero), turned strongly positive for much of March, and ended the month neutral, averaging positive for the month.
- Teleconnections (influence on weather): To the extent teleconnections are known, a positive AO this time of year (February-April) is typically associated with wet conditions in coastal Washington and the Tennessee to Ohio valleys, dryness in the Central Plains, Oregon to California, and much of the Atlantic coast to mid-Gulf of Mexico Coast, above-normal temperatures east of the Rockies, and upper-level circulation anomalies which are below normal over northern Canada and above normal over the eastern to south central CONUS.
- Comparison to Observed: The March 2014 upper-level circulation anomaly pattern is similar to that expected with a positive AO over northeastern Canada, but does not agree over the rest of North America or the North Pacific. The precipitation anomaly pattern agrees in coastal Washington and the Central Plains, but nowhere else. The monthly temperature anomaly pattern is completely opposite that expected with a positive AO.
- The North Atlantic Oscillation (NAO) pattern
- Description: The NAO teleconnection pattern relates upper-level circulation over the North Atlantic Ocean to circulation features over the Northern Hemisphere mid-latitudes.
- Status: The daily NAO index was positive throughout March, ending the month near zero, and averaged positive for the month.
- Teleconnections (influence on weather): To the extent teleconnections are known, a positive NAO during this time of year (April on the teleconnection maps) is associated with warmer-than-normal temperatures across most of the CONUS, near normal temperatures over Alaska, drier-than-normal conditions across the Southeast and in parts of the West and Central Plains, near normal precipitation across Alaska, and positive upper-level circulation anomalies across the CONUS.
- Comparison to Observed: The March 2014 monthly temperature anomaly pattern agrees in the Southwest but is opposite that expected across the rest of the CONUS. The precipitation anomaly pattern agrees over the Central Plains but not elsewhere. The upper-level circulation anomaly pattern matches over the Southwest, but not elsewhere.
- The West Pacific (WP) pattern
- Description: The WP teleconnection pattern is a primary mode of low-frequency variability over the North Pacific and reflects zonal and meridional variations in the location and intensity of the (East Asian) jet stream in the western Pacific.
- Status: The monthly WP index was slightly negative for the month, with the three-month average WP index negative as well.
- Teleconnections (influence on weather): To the extent teleconnections are known, a negative WP during this time of year (April on the maps) is typically associated with above-normal temperatures in the Southwest, below-normal temperatures in the Northern Plains to Midwest, wetter-than-normal weather in coastal Washington (although the teleconnections in general are very weak), above-normal circulation anomalies over the Southwest, and below-normal circulation anomalies over the Northern Plains.
- Comparison to Observed: The March 2014 monthly temperature, precipitation, and upper-level circulation anomaly patterns are a good match to a negative WP in the CONUS where teleconnections exist.
- The East Pacific-North Pacific (EP-NP) pattern
- Description: The EP-NP teleconnection pattern relates SST and upper-level circulation patterns (geopotential height anomalies) over the eastern and northern Pacific to temperature, precipitation, and circulation anomalies downstream over North America. Its influence during the winter is not as strong as during the other three seasons.
- Status: The SST pattern over the northeastern North Pacific during March 2014 saw a continuation of the warmer-than-normal SSTs there, but the magnitude of the warmth continued to decrease. The monthly EP-NP index has been positive since October, pulling the 3-month running mean well into positive territory. The March value was also positive. The positive phase of the EP-NP pattern is associated with above-average surface temperatures over the northeastern Pacific (thus, the negative phase is associated with below-normal SSTs).
- Teleconnections (influence on weather): To the extent these teleconnections are known, a positive EP-NP index during this time of year (April on the maps) is typically associated with cooler-than-normal temperatures across the U.S. east of the Rockies, warmer-than-normal temperatures along the immediate West Coast and in Alaska, wetter-than-normal conditions in the vicinity of Wyoming and parts of the Ohio Valley (although the precipitation teleconnections are weak), below-normal upper-level circulation anomalies (stronger upper-level trough) over the eastern half of Canada and the northeastern CONUS, and above-normal upper-level circulation anomalies (stronger upper-level ridge) over western Canada and Alaska and extending weakly into the western CONUS.
- Comparison to Observed: The March 2014 temperature anomaly pattern is a very good match over the CONUS and western Alaska. The precipitation anomaly pattern is a reasonable match in Wyoming. The upper-level circulation anomaly pattern matches over eastern Canada and the northeastern and southwestern CONUS, but is displaced to the north over western Canada and Alaska.
Examination of these circulation indices and their teleconnection patterns, and comparison to observed March 2014 temperature, precipitation, and circulation patterns, suggest that the weather over the CONUS in March was most closely related to the jet stream and ocean-atmosphere interactions over the North Pacific Ocean. ENSO was neutral, and thus not a player. Slowly-evolving atmospheric and oceanic forcings in the western Pacific weakened the influence of the MJO, but the MJO may have exerted some influence on precipitation during the first half of the month. The teleconnections for the NAO match March's temperature and circulation patterns in the Southwest and precipitation patterns in the Central Plains, but not elsewhere, so the match could have been coincidence. The PNA and AO indices show essentially no significant correlation to the March weather patterns. It is the North Pacific indices which have the best correlations — EP-NP and WP with the March temperature and circulation anomaly patterns. The lack of agreement with the EP-NP upper-level circulation teleconnections in western Canada and the Pacific Northwest probably reflects the undercutting of the western North America ridge by moist short-wave troughs. The EP-NP and WP show some agreement with the precipitation anomaly pattern where teleconnections exist, but so do the MJO, PNA, and NAO. It should be noted that precipitation relationships with these indices are generally weak. This month illustrates how the anomaly patterns can be strongly represented by the indices (EP-NP, WP) measuring one atmospheric driver (the North Pacific Ocean) for circulation and temperature but also have elements of several other drivers (or modes of atmospheric variability) for precipitation.